在下列定语从句中，填入适当的介词。
1. The pencil ______ which he wrote was broken.
2. He built a telescope ______ which he could study the skies.
3. The woman, ______ whom I learned the news, is a nurse.
4. The wolf ______ which the sheep was killed was shot.
5. She has three children, all ______ whom are at school.
6. There is a tall tree outside, ______ which stands our teacher.
7. My glasses, ______ which I was like a blind man, fell to the ground and broke.
8. In the dark street, there wasn't a person ______ whom she could turn for help.
9. The age ______ which children can go to school is seven.
10. His bike ______ which he went to work was stolen last night.
11. Do you know the building ______ which is flying a red flag?
12. Ours is a beautiful school, ______ which we are proud.
13. The desk ______ which Jack is leaning is John's.
14. He bought a book yesterday, the author ______ which is a teacher.